1. Field of the Invention
This invention relates broadly to gravity backwash filters and gravity filtering and backwashing methods. More particularly, it concerns so-called variable declining rate filtration systems and methods.
2. Description of the Prior Art
Granular media filters used for the removal of suspended material from water, waste water, or other fluids have been operated in many modes such as constant pressure, constant rate, constant level, influent flow splitting and variable declining rate filtration (VDRF). Of these, VDRF offers desirable advantages (see the EPA technology transfer seminar publication entitled "Wastewater Filtration Design Considerations" of July 1974, pages 28-32). The present invention provides improvement in VDRF apparatus and methods.
The construction of VDRF filters has taken many forms, the majority of which use valves to control flows during the filtering and backwashing cycles. However, the use of siphons is advantageous because of their simpler mechanical construction and reliability.
Filter apparatus using siphons for flow control is described in U.S. Pat. Nos. 3,134,735; 3,282,432; 3,312,348 and 4,122,013. However, in such apparatus, only influent flow splitting operation is possible limiting the usefulness thereof. A different application of siphons having several unique features in accordance with the present invention retains the advantages of siphons and allows the use of VDRF operation.
The aforesaid EPA publication on page 30, shows the conventional configuration for the VDRF type filter in which the effluent discharge level control weir is below the wash trough neccessitating the use of a backwash supply under pressure, usually a pumped supply. However, it is known to have an effluent weir above the wash trough thereby providing without use of pumps the necessary pressure for backwashing (see U.S. Pat. Nos 3,771,655 and 4,537,687). The present invention uses this type of wash pressure in providing new advancements in the filtering art.
Another problem encountered with declining rate filters is the need to restrict the influent flow rate to the design maximum of the filter during periods of low media headloss following a backwash. In the past, this has been attained by (a) use of a single inlet siphon equipped with flow restricting means, (b) flow restricting means at the outlet of the filter basin or (c) a partializing control on a single influent siphon. The present invention provides new, improved means for restricting influent flow rate in declining rate filters during periods of their operation under low media headloss.